1. Field of the Invention
The present invention relates to a process of producing an optical element such as a circular-polarization-controlling element, and particularly relates to a process of producing an optical element by the use of a radiation-curing liquid crystalline material having cholesteric regularity, and to an optical element.
2. Description of Related Art
Optical elements comprising liquid crystal layers having cholesteric regularity (cholesteric layers) are widely used as circular-polarization-controlling elements (circularly polarizing plates, color filters, etc.) for use in liquid crystal displays.
To produce an optical element, such as a circularly polarizing plate that reflects all visible light, a reflection-type color filter on which each pixel is composed of regions having selective reflection wave ranges equal to the wave ranges of red (R), green (G) and blue (B) colors, or an optical element that is used in a transmission or semi-transmission liquid crystal display in order to improve light utilization efficiency, it is necessary to form a cholesteric layer having a broadened selective reflection wave range or a cholesteric layer having selective reflection wave ranges controlled to be equal to the wave ranges of red, green and blue colors. For this reason, there has been demanded a method of controlling the selective reflection wave range of a cholesteric layer with ease and high precision.
To fulfil this demand, the following methods have been proposed so far: (1) a method in which optically active groups composing the cholesteric structure of a cholesteric layer are modified or deactivated to change the selective reflection wave range of the cholesteric layer (Japanese Laid-Open Patent Publication No. 54905/1998), and (2) a method in which a liquid crystal layer having cholesteric regularity is brought into contact with a solvent or solvent mixture to broaden its selective reflection wave range (Japanese Laid-Open Patent Publication No. 316755/1998).
However, the above two methods are disadvantageous as described below. In the method (1), in which optically active groups in a cholesteric layer are modified or deactivated, the modified or deactivated molecules become impurities to lower the stability of the cholesteric layer itself. If such a cholesteric layer is incorporated into a liquid crystal display, the display cannot clearly display an image. With the method (2), on the other hand, only a cholesteric layer having lowered intensity of color is obtained. If such a cholesteric layer is incorporated into a liquid crystal display, the display cannot clearly display an image.